System and Method For Measuring and Analyzing Target Emissions

ABSTRACT

An interrogation component is controllable between three detection/timing modes: neutron generator OFF for predetermined amount of time (Mode 1), neutron generator pulsing at 5-10 kHz/2-10 microseconds (Mode 2), and neutron generator pulsing at 200-400 Hz/25-200 microseconds (Mode 3). Utilizing the interrogation component in the three detection/timing modes to inspect a target facilitates data collection in both passive and active modes for both passive and stimulated emissions of gamma and neutron radiation.

CROSS-REFERENCE TO RELATED APPLICATIONS

The current application is a divisional application of and incorporatesby reference in its entirety U.S. application Ser. No. 11/033,552entitled SYSTEM AND METHOD FOR MEASURING AND ANALYZING TARGET EMISSIONSfiled Jan. 12, 2005, which claims the benefit of priority to U.S.Provisional Patent Application No. 60/602,041 entitled METHODS FORANALYZING TARGET EMISSIONS filed Aug. 17, 2004, also incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention related generally to the field of target inspection todetermine the contents thereof through radiation detection. Theinvention relates more specifically to the use of inspection systems andmethod.

2. Description of the Related Art

The installation of cargo inspection systems has been instrumental inreducing the flow of illicit contraband into the United States. Althoughsuch systems were primarily installed for the detection of drugs, suchsystems have also contributed to stemming the occurrence of humantrafficking and the flow of stolen vehicles. The role of currentinspection systems is now being amplified as they are asked to not onlylook for illicit drugs but also for explosives and weapons of massdestruction. These cargo inspection systems are “anomaly” detectors.Currently, when an anomaly is detected, it can only be identifiedthrough an intrusive manual inspection which is inherently limited bythe ability, condition and initiative of inspection personnel. This is acostly and time consuming operation which exposes inspection personnelto serious risks and has resulted in increasing delays at bordercrossings. Often it lacks the specificity to clearly identify the natureof the anomaly necessitating additional resources and/or the destructionof unverifiable suspect items. There is a need for companion devicesthat can provide identification of anomalies in a reliable, rapid andnon-intrusive manner.

SUMMARY OF THE INVENTION Summary of the Problem

There is a current need for the ability to inspect targets, e.g.,vehicles, cargo and the like, arriving at ports of entry and othersimilar locations to secure the movement of freight to and from variouscountries. More particularly, explosives and weapons of mass destructionand the fundamental building blocks thereof, i.e., fissionablematerials, are increasingly being trafficked between countries. Whilethe gamma signature of unshielded fissile and fissionable materials canbe easily observed by commonly available passive detector systems, thissignature can be shielded with modest amounts of lead. For example, bothHEU and weapons grade Plutonium (WGPu) have relatively low gamma rayemission rates that have an average low energy spectrum that is easilyshielded. In contrast, neutron emissions from fissile materials,primarily from the ²⁴⁰Pu content in WGPu is virtually unaffected byequivalent amounts of lead shielding. Therefore, passive gamma-ray andneutron measurements are necessary and important but not sufficienttools for counter-terrorism and nuclear security applications.

While all nuclear material is applicable to detection with activeinterrogation, HEU has been demonstrated to be much more difficult todetect than plutonium due to its very low spontaneous neutron emissionsand low-energy gamma-ray emissions. Passive gamma measurements ofshielded HEU are very difficult to detect.

Summary of the Solution

The solution system and method described herein are based on a neutroninterrogation with multiple-detector configuration that may be used toinspect targets, e.g., cargo and vehicles, for prohibited materialsincluding, inter alia, explosives, chemical warfare agents, illicitdrugs and special nuclear materials (SNM).

In a first embodiment of the present invention, an interrogationcomponent for inspecting the contents of a target is described. Theinterrogation component includes a neutron generator capable ofgenerating neutron pulses at a first frequency and at a second frequencyand directing the first and second frequency neutron pulses at thetarget; a first detector configured to detect a first type of radiationemitted from the target in response to the first frequency neutronpulses; and a second detector configured to detect a second type ofradiation emitted from the target in response to the second frequencyneutron pulses.

In a second embodiment of the present invention, an interrogation systemfor inspecting a target is described. The interrogation system includesa first interrogation component including (i) a first neutron generatorcapable of generating neutron pulses at a first frequency and at asecond frequency and directing the first and second frequency neutronpulses at the target; (ii) a first detector configured to detect a firsttype of radiation emitted from the target in response to the firstfrequency neutron pulses; and (iii) a second detector configured todetect a second type of radiation emitted from the target in response tothe second frequency neutron pulses. The system further includes asecond interrogation component including: (i) a second neutron generatorcapable of generating neutron pulses at a first frequency and at asecond frequency and directing the first and second frequency neutronpulses at the target; (ii) a third detector configured to detect a firsttype of radiation emitted from the target in response to the firstfrequency neutron pulses; and (iii) a fourth detector configured todetect a second type of radiation emitted from the target in response tothe second frequency neutron pulses. The first and second interrogationcomponents are located on opposite sides of the target.

In a third embodiment of the present invention, a method ofinterrogating the contents of a target is described. The method includesdirecting neutron pulses having a first frequency at the target;detecting a first type of radiation emitted from the target in responseto the neutron pulses at the first frequency; directing neutron pulseshaving a second frequency at the target; and detecting a second type ofradiation emitted from the target in response to the neutron pulse atthe second frequency.

BRIEF DESCRIPTION OF THE FIGURES

In the Figures:

FIGS. 1 a and 1 b show an interrogation component according to anembodiment of the present invention;

FIG. 2 shows a prior art neutron die away curve for depleted uranium;

FIG. 3 shows a neutron generator firing sequence according to anembodiment of the present invention;

FIGS. 4 a and 4 b show target inspection systems according toembodiments of the present invention;

FIG. 5 shows a neutron capture vs. time analysis for elementalidentification according to an embodiment of the present invention;

FIG. 6 shows a neutron generator firing sequence according to anembodiment of the present invention; and

FIG. 7 shows process steps for data analysis according to an embodimentof the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE PRESENTINVENTION

Referring to FIGS. 1 a and 1 b, a preferred target inspection systemincludes at least one detector/source interrogation component (hereafter“interrogation component”) 10 that includes gamma radiation detectors 12and neutron detectors 14 and at least one neutron source 16 on a heightadjustable pillar 18. In a particular embodiment of the interrogationcomponent 10, four gamma radiation detectors 12 are placed at thecorners of a square, e.g., approximately 3 feet from each other. Eachdetector 12 includes by a cylindrical radiation shield 13 b made of,e.g., lead or tungsten, to minimize the gamma rays coming from thebackground. Detection material 13 a is recessed approximately 2 to 3inches inside the radiation shield 13 b to limit the area seen by thedetector, thus increasing the signal to background ratio. Further,because the neutrons produced from the neutron generator are emittedisotropically in all directions, radiation shielding (trapezoid-shaped)15 made of a combination of materials such as iron, lead, polyethylene,and borated polyethylene is placed between each detector and the neutrongenerator.

In a specific embodiment, the neutron generator 16, e.g., operating at14 MeV, is configured to pulse neutrons approximately 2-10 microsecondswide in a first frequency range of approximately 5,000-10,000 Hz, inorder to excite and detect gamma radiation from a first class ofprohibited materials, e.g., explosives, chemical warfare agents, andillicit drugs. The emitted gamma radiation is detected with a first setof detectors 12 and subjected to chemical elemental analysis. A detaileddescription of an exemplary chemical elemental analysis method isdescribed below and in U.S. Pat. Nos. 5,982,838 and 6,563,898 which areincorporated herein by reference in their entirety. The same neutrongenerator 16 also uses pulsed neutrons that are approximately 25-200microseconds wide in a second frequency range, i.e., 200-400 Hz, inorder to excite and detect neutron radiation from a second class ofprohibited materials, e.g., special nuclear materials such as ²³⁵U or²⁴⁰Pu, with a second set of detectors 14 using a differential die-awaytechnique (DDAT). These stimulated neutrons emanating from the targethave detectable characteristic decay curves which are detectable by theneutron detectors 14, e.g., Li-6 doped glass fibers, He-3 counters orthe like, even when the fissionable materials are being intentionallyshielded within the target. By way of example, the preferred targetinspection system and method is capable of differentiating between 3 gand 6 g of ²³⁵U (1 kg and 2 kg of depleted uranium, respectively) asshown in FIG. 2 from the prior art.

Additionally, the interrogation component may also be utilized in apassive mode in order to detect gamma and/or neutron radiation withgamma detectors 12 and neutron detectors 14 that are being emitted fromthe target without the need for active interrogation. More particularly,referring to FIG. 3, in a preferred embodiment, the interrogationcomponent is controllable between three detection/timing modes: neutrongenerator OFF for predetermined amount of time (Mode 1) 22, neutrongenerator pulsing at 10 kHz/10 microseconds (Mode 2) 20, and neutrongenerator pulsing at 200-400 Hz/25-200 microseconds (Mode 3) 24.Utilizing the interrogation component in the three detection/timingmodes to inspect a target facilitates data collection in both passiveand active modes for both passive and stimulated emissions of gamma andneutron radiation.

Given the fact that the chemical elements and several elemental ratiosare quite different for innocuous substances, drugs, explosives,chemical weapon agents, etc., the system and method described herein areapplicable in a variety of situations, e.g., identification of filler ofshells, differentiating chemical agents from innocuous or high explosivefillers, confirming buried landmines etc. With respect to Mode 2, theneutron generator, e.g., pulsed deuterium-tritium (d-T), generatesneutrons upon application thereto of a DC high voltage (of the order of100 kV) between the cathode and a tritiated target. Deuterium atoms areemitted from the cathode when it is heated. These atoms are then ionizedand accelerated in a high voltage field of up to 100 kV, to impinge ontritium atoms in the target. The fusion of the deuterium and tritiumnuclei creates neutrons with energy of 14 MeV. The deuteron beam ispulsed by applying a gated, e.g., 2-3 kV, voltage between the cathodeand an intermediate electrode. This type of neutron generator allows forthe production of neutrons “on demand.” Since the neutrons are onlyproduced when the high voltage is applied on the generator, when thereis no high voltage, there is no external radioactivity.

The neutrons generated by neutron generator 16 can initiate severaltypes of nuclear reactions ((n,n′γ), (n,pγ), (n, γ) etc.) on the targetunder interrogation. The gamma (y) rays from these reactions aredetected by the gamma ray detectors 12 which may be, for example,bismuth germanate (BGO) or NaI scintillators. During the Mode 2 neutronpulse, the gamma-ray spectrum is primarily composed of gamma rays fromthe (n,n′γ) and (n,pγ) reactions of fast neutrons with elements such asC and O. This spectral data is stored at a particular memory locationwithin a data acquisition system (not shown). Between pulses, some ofthe fast neutrons that are still within the target lose energy bycollisions with light element composing the target. When the neutronshave energy less than 1 eV, they are captured by such elements as H, N,Cl and Fe through (n, γ) reactions. The gamma rays from this set ofreactions are detected by the same set of detectors 12 but stored at adifferent memory address within the data acquisition system (not shown).This procedure is repeated with a frequency of approximately 10 kHz.After a predetermined number of pulses, there is a longer period thatallows the detection of gamma rays emitted from elements such as Si andP that have been activated. Therefore, by utilizing fast neutronreactions, neutron capture reactions, and activation, a large number ofelements contained in a target can be identified through the targetinspection system operating in Mode 2. FIG. 5 shows the time sequence ofthe nuclear reactions taking place.

Further to Mode 2, during the neutron pulse, the gamma ray spectrum isprimarily composed of gamma rays from inelastic scattering and neutroncapture reactions and is stored at a particular memory location withinthe data acquisition system. These reactions happen immediately and areeliminated as soon as the neutron generator is turned off. Betweenpulses, some of the fast neutrons are slowed down and thermalized, andmay eventually be captured by nuclei in the target, producing gammaswith different characteristic energies. The gamma detectors 12 are gatedso that two spectra, a fast and a thermal spectrum, are acquiredindependently. As referenced above, in Mode 2, the neutron pulseduration is 10 microseconds with a frequency of 10 KHz. The spectra areacquired by counting for 2 to 5 minutes.

High explosives, e.g., TNT, RDX, C-4, etc., are composed primarily ofthe chemical elements hydrogen, carbon, nitrogen, and oxygen. Similarly,illicit drugs are typically composed of high amounts of hydrogen andcarbon and, in many instances, show a strong chlorine signature. Manyinnocuous materials are also primarily composed of these same elements,but the elemental ratios and concentrations are unique to each material.Table 1 set forth below shows the atomic density of elements for variousmaterials along with the atomic ratios. The problem of identifyingexplosives and illicit drugs is thus reduced to the problem of elementalidentification. Nuclear techniques show a number of advantages fornondestructive elemental characterization. These include the ability toexamine bulk quantities with speed, high elemental specificity, and nomemory effects from the previously measured target.

TABLE 1 Density Or Ratio H C N O Cl C/O C/N Cl/O Narcotics High High LowLow Medium High > 3 High Very High Explosives Low- Med High Very MediumLow < 1 Low > 1 Low to Medium High to None Medium Plastics Medium- HighHigh Medium Medium Medium Very — High to Low to None High

Neutrons have high penetrability and can traverse with ease the part ofthe target volume behind which a suspected anomaly is to beinterrogated. The incident neutrons interact with the nuclei of thevarious chemical elements in the anomaly, emitting characteristic gammarays which act as the fingerprints of the various chemical elements. Thegamma rays are collected by gamma ray detector(s) 12, capable ofdifferentiating them according to their energy and their quantity ateach energy level. The incident neutrons interact with the chemicalelements in the anomaly and also with surrounding target material. Theseinteractions result in gamma rays that constitute the background of themeasurement.

The chemical elements of interest for the detection of illicit drugs,explosives, etc., require different neutron energies in order to beobserved. Elements such as H, Cl, and Fe are best observed throughnuclear reactions initiated from very low energy neutrons. Otherelements such as C and O need neutron energies of several MeV to beobserved at all. To satisfy this, a neutron source is required that canproduce the high-energy neutrons for measurement of elements such as Cand O, and low energy neutrons (energy <0.025 eV) for elements such as Hand Cl. Such a task can be accomplished with the use of a pulsed (d,T)neutron generator.

With respect to Mode 3, the neutron generator produces neutrons, i.e.,frequency of 200-400 Hz for between 25-200 μs, that interact withfissionable materials within a target, e.g., ²³⁵U and ²⁴° Pu. At the endof the of the neutron pulse neutrons emitted by any fissionablematerials will be detectable by neutron detectors and identifiable dueto their characteristic decay curves.

Depending on the size of the target, it may be necessary to utilize morethan one interrogation component in a preferred target inspectionsystem. For example, referring to FIG. 4 a, because of the 4 footeffective range of the interrogation, there may be two interrogationcomponents 10 a and 10 b utilized to inspect a vehicle, i.e., one oneach side of the interrogated vehicle 30. With respect to thisembodiment, although the detectors 12 a and 14 a (not explicitly shown)within interrogation component 10 a will be recessed and shielded asdescribed above from the neutron generator 16 a (not explicitly shown),each of the detectors 12 a and 14 a within interrogation component 10 awill inadvertently be exposed to the neutron flux coming from theneutron generator 16 b (not explicitly shown) within interrogationcomponent 10 b on the other side of the vehicle. And vice versa fordetectors 12 b and 14 b (not explicitly shown). This will result in anunwanted increase of the gamma-ray background. To avoid this, the firingsequence of the two neutron generators will be controlled so that eachside's detectors will not be accumulating while the other side's neutrongenerator is on. An exemplary timing sequence is shown in FIG. 6 whereinduring a first time T1, e.g., 10 μs along the “TIME” continuum, thefirst neutron generator in Mode 2 16 a is ON and fast data acquisitionfor first neutron generator 16 a is ON 40, while the second neutrongenerator 16 b, is OFF and no data is being acquired therefore 50.During a time T2, e.g., 40 μs, the first and second neutron generatorsare OFF and thermal data acquisition for both is ON 45. During a timeT3, e.g., 25 μs, the first neutron generator in Mode 3 is ON 48, whilethe second neutron generator remains OFF 50. During a time T4, the firstand second neutron generators are OFF while neutron emission data isbeing acquired 45. During time T5, the first neutron generator is OFF50, while the second neutron generator in Mode 2 is ON 40. Time T6follows Time T2. During time T7, the first neutron generator is OFF 50,while the second neutron generator in Mode 3 is ON 48. Time T8 followstime 4.

Further, in addition to the vertical movement Y of the interrogationcomponent by virtue of the adjustable pillar 18 (FIG. 1) theinterrogation component(s) may be set on translatable platform(s) 19 soas to control horizontal movement X of the interrogation component 10towards and away from the target 30. Further still, in an alternativeembodiment, additional detectors, i.e., gamma and neutron, may beutilized in conjunction with the interrogation component(s) 10 in orderto accommodate different target sizes while maximizing the signal tonoise ratio.

In a further embodiment of the present invention, the target inspectionsystem further includes a threshold anomaly detection system and method,wherein gamma source/gamma detector and/or x-ray source/x-ray detectorconfigurations (hereafter “gamma/x-ray interrogator”) are first utilizedin order to identify the mere presence of an anomaly in a target duringan initial scan. Referring to FIG. 4 b, a two phase target inspectionsystem includes an initial scan of the target 30 using a gamma/x-rayinterrogator 35 and a secondary scan using the interrogation components10 described herein to identify the particulars of an anomaly once thepresence of an anomaly has been detected during the initial scan.Various embodiments of appropriate gamma/x-ray interrogatorconfigurations are described in U.S. Pat. No. 6,507,025, entitled,“DENSITY DETECTION USING REAL TIME DISCRETE PHOTON COUNTING FOR FASTMOVING TARGETS,” U.S. patent application Ser. No. 10/767,723, entitled“METHOD AND SYSTEM FOR AUTOMATICALLY SCANNING AND IMAGING THE CONTENTSOF A MOVING TARGET,” filed Jan. 30, 2004 by Richardson et al., U.S.patent application Ser. No. 10/717,632, entitled, “SYSTEM AND METHOD FORTARGET INSPECTION USING DISCRETE PHOTON COUNTING AND NEUTRON DETECTION,”filed Nov. 21, 2003, by Verbinski et al.; U.S. patent application Ser.No. 10/833,131 entitled, “DENSITY DETECTION USING REAL TIME DISCRETEPHOTON COUNTING FOR FAST MOVING TARGETS,” filed Apr. 28, 2004, byVerbinski et al., which is a continuation of U.S. patent applicationSer. No. 09/925,009, entitled, “DENSITY DETECTION USING REAL TIMEDISCRETE PHOTON COUNTING FOR FAST MOVING TARGETS,” filed Aug. 9 2001, byVerbinski et al.; all of the aforementioned patent applications are alsoincorporated herein by reference in their entireties.

Referring to FIG. 7, in a first exemplary processing embodiment, oncethe gamma and neutron emission data is collected by the detectors of theinterrogation component S10, the spectra data 60 is processed by thetarget inspection system to identify contraband hidden among innocuousobjects within the target. The spectra data are analyzed using a primaryanalysis application by performing a least squares analysis which uses alibrary of gamma (and neutron) spectra for several chemical elementsthat are expected to be either on the background (e.g. other parts ofthe target) or in the target to be interrogated S20. The least squaresanalysis (“LSA”) method does not rely on any specific chemical element.Instead, it utilizes all chemical elements that are present or, incertain cases, absent from a spectrum. Utilizing spectra data 60, basedon the assumed linear model, a generic spectrum |S

can be written as follows:

|S

=c ₁ |R ₁

+c ₂ |R ₂

+ . . . +c _(n) |R _(n)

where the c_(i) are coefficients and |R₁

. . . |R_(n)

are generalized responses (in particular |R₁

is the background). The equation above can be written as a matrix vectorproduct, as follows:

|S

=R|c

where R is a known matrix. If R were a square matrix the solution wouldbe

|c

R ⁻¹ |S

.

Since R is not a square matrix (R may have four or five columns andhundreds of lines, corresponding to the channels in the spectrum), thesystem of equations is over determined. The matrix R^(T)R (where Tindicates the transpose) is a square matrix, and the system can besolved in the least squares sense, as follows:

|c

=(R ^(T) R)⁻¹ R ^(T) |S

The vector |c

contains the indicators based on the LSA method. The LSA primaryanalysis application provides the results in counts/second for eachchemical element of importance 62. These results are then used in asecondary analysis application to identify the target S30, 64.

In order to maximize the reliability of the results 64 from the targetinspection system, the system must utilize a secondary analysisapplication that maximizes probability of detection and minimizesprobability of false alarm. Applying The Generalized Likelihood RatioTest (GLRT) process (S30) to the detected data 62 in conjunction withtraining data sets for known materials, the target inspection systemoptimizes the probability of detection and minimizes the probability offalse alarms. More particularly, the training data sets are based on anelemental concentrations data bank established by interrogating a largenumber of innocuous objects as well as drugs, explosives, WMD, etc.,i.e., known targets. Based on elemental concentrations, as well asseveral elemental ratios, the target inspection system is “trained” todistinguish contraband from innocuous objects. Drugs, for example, areprimarily distinguished through the elements H, C, and Cl; ChemicalWarfare Agents through C, H, O, Cl, S, P, etc. The GLRT approach is astatistical analysis tool that allows one to do hypothesis testing basedon a ratio of two likelihoods: the likelihood that the data point of thetarget being evaluated is inert and the likelihood that the target is areal threat or contraband item, i.e., explosive. To apply the tool oneneeds a set of representative data to train on, after which one selectsa threshold and applies the tool to the stream of data points thatfollow. By moving the threshold one can develop a ROC (ReceiverOperating Characteristic) curve, which is a good indicator of thedetection capability of the system.

In operation, for each sample the indicators are known and it is knownwhether the sample is explosive or inert. By way of example, using thedata 60 from the LSA method, a sample may consist of a vector W with 4components (C, H, N, and O intensities, obtained from the primaryanalysis application S20). The samples could include all themeasurements, or a subset, for example all the measurements made on aparticular kind of environment, such as on a concrete surface. Wecompute the mean and the covariance matrix for the explosives (μ₁, Cov₁)and for the inert items (μ₀, Cov₀), respectively. For a generic vector Wcompute

λ=(W−μ ₁)^(T)(Cov ₁)⁻¹(W−μ ₁)−(W−μ ₀)^(T)(Cov ₀)⁻¹(W−μ ₀)

where T indicates the transpose. The quantity λ can be used to make adeclaration, by comparing its value to a threshold level. The procedurefor the declaration is as follows:

-   If λ<Threshold the declaration is explosive.-   If λ>Threshold the declaration is inert.    By comparing the result of the declarations to the known state    (explosive or inert) of the sample we can calculate the detection    probability (DP) and the probability of false alarms (FA) for that    particular threshold. If we let the threshold vary over the entire    range of λ we obtain a ROC (receiver operating characteristic)    curve, which is a plot of detection probability versus probability    of false alarms. The ROC curve is a global way of assessing the    performance of both primary and secondary analysis applications. The    GLRT method can then be used for making decisions on new data. A    threshold value that corresponds to an acceptable level on the ROC    curve (for example 10% FA, 80% DP) is selected, and the same    procedure described above is used for making the decision. The    result of the decision may be explosive or inert. The GLRT parameter    may also be used to calculate a confidence value to associate with    the declaration. If enough information is available for a range of    different substances, the GLRT parameter can also be used for    substance identification. The substance is identified as belonging    to the class (either explosive or inert) corresponding to the    highest confidence.

Still referring to FIG. 7, in a second exemplary embodiment, the primaryanalysis application may utilize principal component analysis (“PCA”)which is based on a particular expansion in terms of orthonormalfunctions. The PCA method relies on general features of the accumulatedspectrum and not on the particular chemical elemental content of the“anomaly” under interrogation. This approach is completely heuristic andas such it does not require any auxiliary measurements or underlyingmodels. Every spectrum (or the difference between signal and background)can be represented as a weighted sum of basis vectors. If an adequaterepresentation of a spectrum can be attained with only a small number ofbasis vectors, the coefficients of the expansion form a feature vectorthat can also be used to characterize the sample. One advantage of thePCA method is that it is self-consistent within one data set and doesnot require auxiliary measurements outside that data set. This meansthat, where the system is well trained, it is possible to analyze atarget without relying on spectral de-convolution.

More particularly, any vector, including a spectrum |S

, can be decomposed into a sum of vectors as follows:

|S

=c ₁ |P ₁

+c ₂ |P ₂

+ . . . +c _(n) |P _(n)

where the c₁ . . . c_(n) coefficients are numbers, and the vectors |P₁

. . . |P_(n)

form an orthonormal basis. The above equation is true for anyorthonormal set. The question is, is there a preferred way to choose the|P_(i)

? When we have many vectors|S

we can arrange them to form a matrix X. This matrix is not square,however the matrix X^(T)X that is proportional to the covariance of thematrix X (provided that the data has been mean centered) is square. Theeigenvectors of this square covariance matrix by definition form anorthonormal set. They can be ordered in descending order according tothe magnitude of the corresponding eigenvalues. The advantage of thisprocedure is that often one does not need all of the eigenvectors toexpand the spectra. Instead, a relatively small number of components maybe sufficient for the spectral expansion, and most of the variance inthe data is captured by the first few principal components. Once theprincipal components have been determined, a particular spectrum isrepresented by a small number of indicators, also known as scores, whichare obtained by projecting the vector onto the principal components, asfollows:

c_(i)=

P_(i)|S

Accordingly, applying the PCA method described above to the spectra data60, a matrix of all the spectra data 60 is formed, including taking intoaccount the background. The covariance of this matrix is calculated. Theset of eigenvectors of the covariance matrix is computed and they areordered in descending order by eigenvalue. A smaller subset ofeigenvectors from the top of the ordered list is selected and these arethe principal components to be used. Finally, the data vector (spectrum)is projected on the above components, thus extracting a group ofindicators, i.e., scores, for each data vector 62. The GLRT secondaryanalysis method is then performed on the data vector scores 62 asdescribed above with respect to the elemental intensities from the LSAmethod.

The embodiments described above are intended to exemplary. One skilledin the art recognizes the numerous alternative components andembodiments which may be substituted for the particular examplesdescribed herein and still fall within the scope of the invention.

1. A method of interrogating the contents of a target comprising:directing neutron pulses having a first frequency at the target;detecting a first type of radiation emitted from the target in responseto the neutron pulses at the first frequency; directing neutron pulseshaving a second frequency at the target; and detecting a second type ofradiation emitted from the target in response to the neutron pulse atthe second frequency.
 2. The method according to claim 1, furthercomprising directing gamma radiation at the target and detecting atleast a portion of the gamma radiation that passes through the target.3. The method according to claim 1, further comprising detecting a thirdtype of radiation emitted from the target that is not response tointerrogation by the neutron pulses.